U.S. patent number 11,394,221 [Application Number 16/430,576] was granted by the patent office on 2022-07-19 for method and system for controlling dc bus voltage.
This patent grant is currently assigned to Delta Electronics, Inc.. The grantee listed for this patent is Delta Electronics,Inc.. Invention is credited to Jian Li, Changliang Liu, Yunfeng Liu, Guoqiao Shen, Guojin Xu, Jinfa Zhang.
United States Patent |
11,394,221 |
Shen , et al. |
July 19, 2022 |
Method and system for controlling DC bus voltage
Abstract
A method for controlling a DC bus voltage in a DC bus system,
the system including a DC bus and an energy storage unit coupled to
the DC bus, includes: detecting a DC bus voltage; detecting, by the
energy storage unit, a DC bus voltage of the DC bus; determining,
by the energy storage unit, a power reference value based on the DC
bus voltage; and adjusting, by the energy storage unit, one of
output power and absorbing power based on the power reference
value.
Inventors: |
Shen; Guoqiao (Taoyuan,
CN), Xu; Guojin (Taoyuan, CN), Liu;
Changliang (Taoyuan, CN), Li; Jian (Taoyuan,
CN), Liu; Yunfeng (Taoyuan, CN), Zhang;
Jinfa (Taoyuan, CN) |
Applicant: |
Name |
City |
State |
Country |
Type |
Delta Electronics,Inc. |
Taoyuan |
N/A |
CN |
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|
Assignee: |
Delta Electronics, Inc.
(Taoyuan, TW)
|
Family
ID: |
1000006442963 |
Appl.
No.: |
16/430,576 |
Filed: |
June 4, 2019 |
Prior Publication Data
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|
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Document
Identifier |
Publication Date |
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US 20190372380 A1 |
Dec 5, 2019 |
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Foreign Application Priority Data
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Jun 5, 2018 [CN] |
|
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201810570503.0 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02J
1/00 (20130101); H02J 7/0068 (20130101) |
Current International
Class: |
H02J
7/00 (20060101); H02J 1/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kessie; Daniel
Assistant Examiner: Baxter; Brian K
Attorney, Agent or Firm: Xu; Qinghong
Claims
What is claimed is:
1. A method for controlling a DC bus voltage of a DC bus system,
the system comprising a DC bus and an energy storage unit coupled
to the DC bus, wherein the method comprises: detecting, by the
energy storage unit, the DC bus voltage of the DC bus system;
determining, by the energy storage unit, a power reference value
based on the DC bus voltage of the DC bus system; and adjusting, by
the energy storage unit, one of output power and absorbing power
based on the power reference value, wherein when the energy storage
unit is configured to output power to the DC bus system, the
determining, by the energy storage unit, the power reference value
based on the DC bus voltage of the DC bus system comprises:
determining the power reference value based on a first preset
operating mode when the DC bus voltage of the DC bus system is
greater than a first preset value and less than or equal to a
second preset value; determining the power reference value based on
a first function when the DC bus voltage of the DC bus system is
greater than the second preset value and less than or equal to a
third preset value; and determining the power reference value based
on a third function when the DC bus voltage of the DC bus system is
greater than a fourth preset value and less than or equal to the
first preset value; wherein independent variables of the first
function and the third function are the DC bus voltage of the DC
bus system, and dependent variables of the first function and the
third function are an output power reference value of the energy
storage unit; and when the energy storage unit is configured to
absorb power from the DC bus of the DC bus system, the determining,
by the energy storage unit, the power reference value based on the
DC bus voltage of the DC bus system comprises: determining the
power reference value based on a second preset operating mode when
the DC bus voltage of the DC bus system is greater than the first
preset value and less than or equal to the second preset value;
determining the power reference value based on a second function
when the DC bus voltage of the DC bus system is greater than the
second preset value and less than or equal to the third preset
value; and determining the power reference value based on a fourth
function when the DC bus voltage of the DC bus system is greater
than the fourth preset value and less than or equal to the first
preset value; wherein independent variables of the second function
and the fourth function are the DC bus voltage of the DC bus
system, and dependent variables of the second function and the
fourth function are an absorbing power reference value of the
energy storage unit.
2. The method according to claim 1, wherein when the energy storage
unit is configured to output power to the DC bus, the determining,
by the energy storage unit, a power reference value based on the DC
bus voltage further comprises: determining the power reference
value as a minimum preset output power when the DC bus voltage is
greater than the third preset value; and determining the power
reference value as a rated output power when the DC bus voltage is
less than or equal to the fourth preset value.
3. The method according to claim 1, wherein when the energy storage
unit is configured to absorb power from the DC bus, the
determining, by the energy storage unit, a power reference value
based on the DC bus voltage further comprises: determining the
power reference value as a maximum preset absorbing power when the
DC bus voltage is greater than the third preset value; and
determining the power reference value as zero when the DC bus
voltage is less than or equal to the fourth preset value.
4. The method according to claim 1, wherein the DC bus system
further comprises a new energy generating unit coupled to the DC
bus, and when the energy storage unit is configured to output power
to the DC bus, the method further comprising: setting an output
power limit for the new energy generating unit based on a fifth
function when the DC bus voltage is greater than the third preset
value, controlling an output power of the new energy generating
unit to be below the output power limit; wherein an independent
variable of the fifth function is the DC bus voltage, and a
dependent variable of the fifth function is the output power limit
of the new energy generating apparatus.
5. The method according to claim 4, wherein the fifth function is a
monotone decreasing function.
6. The method according to claim 1, wherein the DC bus system
further comprises a DC load coupled to the DC bus, and when the
energy storage unit is configured to absorb power from the DC bus,
the method further comprising: unloading the DC load when the DC
bus voltage is less than or equal to the fourth preset value.
7. The method according to claim 1, wherein when the energy storage
unit is configured to output power to the DC bus, the first preset
value Vref1 and the second preset value Vref2 satisfy:
Vref1=(1+K1)*Vp; Vref2=(1+K2)*Vp; wherein Vp represents a rated
operational voltage of the DC bus, K1 and K2 represent any one
value ranging from -30% to 30%, and K2 is greater than K1.
8. The method according to claim 1, wherein when the energy storage
unit is configured to output power to the DC bus, the third preset
value Vref3 satisfies: Vref3=(1+K3)*Vref2 wherein Vref2 represents
the second preset value of the energy storage unit, and K3
represents any one value ranging from 0 to 30%.
9. The method according to claim 1, wherein when the energy storage
unit is configured to output power to the DC bus, the fourth preset
value Vref4 satisfies: Vref4=(1+K4)*Vref1 wherein Vref1 represents
the first preset value of the energy storage unit, and K4
represents any one value ranging from 0 to 30%.
10. The method according to claim 1, wherein when the energy
storage unit is configured to output power to the DC bus, the first
function and the third function are monotone decreasing
functions.
11. The method according to claim 1, wherein when the energy
storage unit is configured to absorb power from the DC bus, the
second function and the fourth function are monotone decreasing
functions.
12. A system for controlling a DC bus voltage of a DC bus system,
comprising: a voltage master controller, coupled to the DC bus
system and configured to adjust the DC bus voltage of the DC bus
system based on a running state of the system; an energy storage
unit coupled to the DC bus system, comprising a power controller
and a power unit, the power controller being configured to adjust
one of output power and absorbing power of the power unit based on
the DC bus voltage of the DC bus system; wherein the power
controller comprises: a detector, configured to detect the DC bus
voltage of the DC bus system; a calculator, configured to determine
a power reference value based on the DC bus voltage of the DC bus
system; and a controller, configured to control an operating state
of the power unit, and adjust the one of output power and absorbing
power of the power unit based on the power reference value, and
wherein when the energy storage unit is configured to output power
to the DC bus system, the calculator is configured to: determine
the power reference value based on a first preset operating mode
when the DC bus voltage of the DC bus s stem is greater than a
first preset value and less than or equal to a second preset value;
determine the power reference value based on a first function when
the DC bus voltage of the DC buss stem is greater than the second
preset value and less than or equal to a third preset value; and
determine the power reference value based on a third function when
the DC bus voltage of the DC bus system is greater than a fourth
preset value and less than or equal to the first preset value;
wherein independent variables of the first function and the third
function are the DC bus voltage of the DC bus system, and dependent
variables of the first function and the third function are an
output power reference value of the energy storage unit; and when
the energy storage unit is configured to absorb power from the DC
bus system, the calculator is configured to: determine the power
reference value based on a second preset operating mode when the DC
bus voltage of the DC bus s stem is greater than the first preset
value and less than or equal to the second preset value; determine
the power reference value based on a second function when the DC
bus voltage of the DC bus system is greater than the second preset
value and less than or equal to the third preset value; and
determine the power reference value based on a fourth function when
the DC bus voltage of the DC bus system is greater than the fourth
preset value and less than or equal to the first preset value;
wherein independent variables of the second function and the fourth
function are the DC bus voltage of the DC bus system, and dependent
variables of the second function and the fourth function are an
absorbing power reference value of the energy storage unit.
13. The system according to claim 12, wherein when the energy
storage unit is configured to output power to the DC bus, the
calculator is further configured to: determine the power reference
value as a minimum preset output power when the DC bus voltage is
greater than the third preset value; and determine the power
reference value as a rated output power when the DC bus voltage is
less than or equal to the fourth preset value.
14. The system according to claim 12, wherein when the energy
storage unit is configured to absorb power from the DC bus, the
calculator is further configured to: determine the power reference
value as a maximum preset absorbing power when the DC bus voltage
is greater than the third preset value; and determine the power
reference value as zero when the DC bus voltage is less than or
equal to the fourth preset value.
15. The system according to claim 12, further comprising a new
energy generating apparatus coupled to the DC bus, wherein when the
energy storage unit is configured to output power to the DC bus,
the calculator is further configured to: limit the power reference
value of the new energy generating apparatus to be below an output
power limit calculated based on a fifth function when the DC bus
voltage is greater than the third preset value, wherein an
independent variable of the fifth function is the DC bus voltage,
and a dependent variable of the fifth function is the output power
limit of the new energy generating apparatus.
16. The method according to claim 15, wherein the fifth function is
a monotone decreasing function.
17. The system according to claim 12, further comprising at least
one of a DC load, wherein when the energy storage unit is
configured to absorb power from the DC bus, the controller is
configured to unload the at least one of the DC load when the DC
bus voltage is less than or equal to the fourth preset value.
18. The system according to claim 12, wherein when the energy
storage unit is configured to output power to the DC bus, the first
preset value Vref1 and the second preset value Vref2 satisfy:
Vref1=(1+K1)*Vp; Vref2=(1+K2)*Vp; wherein Vp represents a rated
operational voltage of the DC bus, K1 and K2 represent any one
value ranging from -30% to 30%, and K2 is greater than K1.
19. The system according to claim 12, wherein when the energy
storage unit is configured to output power to the DC bus, the third
preset value Vref3 and the second preset value Vref2 satisfy:
Vref3=(1+K3)*Vref2 wherein K3 represents any value ranging from 0
to 30%.
20. The system according to claim 12, wherein when the energy
storage unit is configured to output power to the DC bus, the
fourth preset value Vref4 satisfies: Vref4=(1-K4)*Vref1 wherein
Vref1 represents the first preset value of the energy storage unit,
and K4 represents any one value ranging from 0 to 30%.
21. The method according to claim 12, wherein when the energy
storage unit is configured to output power to the DC bus, the first
function and the third function are monotone decreasing
functions.
22. The method according to claim 12, wherein when the energy
storage unit is configured to absorb power from the DC bus, the
first function, the second function and the fourth function are
monotone decreasing functions.
Description
CROSS REFERENCE
This application is based upon and claims priority to Chinese
Patent Application No. 201810570503.0, filed on Jun. 5, 2018, the
entire contents thereof are incorporated herein by reference.
TECHNICAL FIELD
The present disclosure relates to the field of power supply
technologies, and more particularly, to a method and a system for
controlling a direct current (DC) bus voltage.
BACKGROUND
A DC distribution grid has been widely used in, for example, a
subway traction system because of its low line loss, high
reliability, needless phase frequency control, strong ability in
accepting distributed power sources and other advantages.
The subway traction system typically includes a rectifying power
supply, a traction converter (VVVF), an energy feedback grid-tied
inverter system (ERS), a braking energy dissipation resistor system
(EDS), and/or a braking energy storage system (ESS). As shown in
FIG. 1, the ESS system generally controls bidirectional power
conversion by a bidirectional DC converter. The ERS system is used
to feed excessive energy of a DC bus back to the grid. The power of
the VVVF is determined by train running demands and states. To
ensure that a DC bus voltage of a traction DC distribution grid is
stabilized within a normal operating voltage range, it is required
to adjust, in time, power of the ERS, ESS or the EDS based on the
train running states and grid voltage conditions. In related
technologies, a bidirectional power converter of the ESS is
adjusted generally by instructions output by a system controller
(CSU) of the DC distribution grid to control the power of the ESS.
However, when the communication is abnormal or a communication
speed cannot satisfy dynamic power regulation requirements of the
system, it is impossible to guarantee timely regulation of the DC
bus voltage in most cases.
In addition, the DC distribution grid is also used in distributed
new energy power generation and energy storage systems. When the
communication between each energy storage unit (such as ESS and
ERS) and the system controller is abnormal or the communication
speed cannot satisfy requirements for power dynamic change, the bus
voltage instability for the system operation becomes a problem
urgent to be solved.
Therefore, it is required a method for controlling a DC bus voltage
more quickly and more timely.
It is to be noted that the above information disclosed in this
Background section is only for enhancement of understanding of the
background of the present disclosure and therefore it may contain
information that does not form the prior art that is already known
to a person of ordinary skill in the art.
SUMMARY
The present disclosure is directed to provide a method and a system
for controlling a direct current (DC) bus voltage, so as to
overcome, at least to a certain extent, one or more problems caused
by limitations and defects of related technologies.
According to a first aspect of embodiment of the present
disclosure, there is provided a method for controlling a DC bus
voltage in a DC bus system, the system including a DC bus and an
energy storage unit coupled to the DC bus. The method includes:
detecting, by the energy storage unit, a DC bus voltage;
determining, by the energy storage unit, a power reference value
based on the DC bus voltage; and
adjusting, by the energy storage unit, one of output power and
absorbing power based on the power reference value.
According to another aspect of the present disclosure, there is
provided a system for controlling DC bus voltage, which
includes:
a voltage master controller, coupled to a DC bus and configured to
adjust a DC bus voltage based on a running state of the system;
a plurality of energy storage units coupled to the DC bus, wherein
each of the energy storage units comprises a power controller and a
power unit, the power controller is configured to adjust an output
power or an absorbing power of the power unit based on the DC bus
voltage;
wherein the power controller comprises:
a detector, configured to detect the DC bus voltage;
a calculator, configured to determine a power reference value based
on the DC bus voltage; and
a controller, configured to control an operating state of the power
unit to adjust, in real time, the output power or the absorbing
power of the power unit based on the power reference value.
According to the method for controlling a DC bus voltage provided
by the present disclosure, an energy storage unit coupled to the DC
bus automatically adjusts its operating power based on the DC bus
voltage, which may avoid a problem, in the related technologies,
that is limited by a communication state and coordination ability
of a master controller when regulating the DC bus voltage, enhance
efficiency of regulating the DC bus voltage and effectively
guarantee stability of the DC bus voltage.
It should be understood that the above general description and the
detailed description below are merely exemplary and explanatory,
and do not limit the present disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings herein are incorporated in and constitute
a part of this specification, illustrate embodiments conforming to
the present disclosure and together with the description serve to
explain the principles of the present disclosure. Apparently, the
accompanying drawings in the following description show merely some
embodiments of the present disclosure, and persons of ordinary
skill in the art may still derive other drawings from these
accompanying drawings without creative efforts.
FIG. 1 is an architecture diagram of a DC bus in related
technologies:
FIG. 2 is a block diagram of a system for controlling a DC bus
voltage according to an exemplary embodiment of the present
disclosure;
FIG. 3 is a schematic functional diagram of a power controller
according to an exemplary embodiment of the present disclosure;
FIG. 4 is a schematic diagram of determining a power reference
value by a calculator according to an exemplary embodiment of the
present disclosure;
FIG. 5A is a flowchart of a method for controlling a DC bus voltage
according to an embodiment of the present disclosure.
FIG. 5B is a flowchart of a method for controlling a DC bus voltage
according to another embodiment of the present disclosure;
FIG. 6 is a schematic operating diagram when the embodiments of the
present disclosure are applied to a braking energy absorption
system of a subway substation;
FIG. 7 is a schematic diagram of a power control process according
to an embodiment of the present disclosure:
FIG. 8A-FIG. 8C are schematic diagrams of functions in an exemplary
running process of the system as shown in FIG. 6;
FIG. 9 is a block diagram of a DC bus coupled to a new energy
generating apparatus and an energy storage unit according to an
embodiment of the present disclosure; and
FIG. 10 is a schematic diagram of a function in an exemplary
running process according to the embodiment as shown in FIG. 9.
DETAILED DESCRIPTION
Exemplary embodiments will be described more comprehensively by
referring to accompanying drawings now. However, the exemplary
embodiments can be embodied in many forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided so that this disclosure will be made
thorough and complete, and the concept of exemplary embodiments
will be fully conveyed to those skilled in the art. Furthermore,
the described features, structures, or characteristics may be
combined in any suitable manner in one or more embodiments. In the
following description, numerous specific details are provided to
provide a thorough understanding of the embodiments of the present
disclosure. Those skilled in the art will recognize, however, that
the technical solution of the present disclosure may be practiced
without one or more of the specific details described, or that
other methods, components, materials, etc. may be employed. In
other instances, well-known technical solutions are not shown or
described in detail to avoid obscuring aspects of the present
disclosure.
Furthermore, the accompanying drawings are merely schematic
illustrations of the present disclosure. Same or similar parts are
denoted by same reference numbers in the drawings and, thus, a
detailed description thereof will be omitted. Some block diagrams
shown in the figures are functional entities and not necessarily to
be corresponding to a physically or logically individual entities.
These functional entities may be implemented in software form, or
implemented in one or more hardware modules or integrated circuits,
or implemented in different networks and/or processor apparatuses
and/or microcontroller apparatuses.
A detailed description of the exemplary embodiments of the present
disclosure will be made in the following with reference to the
accompanying drawings.
FIG. 2 is a block diagram of a system for controlling a DC bus
voltage according to an embodiment of the present disclosure.
Referring to FIG. 2, the system 200 for controlling a DC bus
voltage may include:
a voltage master controller 21, coupled to a DC bus 20 and
configured to adjust a DC bus voltage based on a running state of
the system:
a plurality of energy storage units 22 coupled to the DC bus,
wherein each of the energy storage units includes a power
controller 221 and a power unit 222, the power controller 221 is
configured to adjust an output power or an absorbing power of the
power unit 222 based on voltage of the DC bus 20.
The voltage master controller 21 may be, for example, one
rectifying power supply or a group of rectifying power supplies,
one bidirectional gird-connected inverter or a group of
bidirectional gird-connected inverters, one primary energy storage
controller or a group of primary energy storage controllers,
etc.
The energy storage unit 22 may be configured to output power output
to the DC bus or absorb power from the DC bus, for example, the ESS
and the ERS as mentioned previously. Taking a subway traction
system as an example, the braking energy storage system (ESS) may
be controlled and adjusted, by a bidirectional DC converter, to be
a power output mode and a power absorbing mode; and the energy
feedback grid-tied inverter system (ERS) may work in the power
absorbing mode, and a power supply may work in the power output
mode. In addition, the energy storage unit may also include a DC
load or an AC load for absorbing power from the DC bus. In an
exemplary embodiment of the present disclosure, each energy storage
unit may be provided with a power controller to adjust an operating
mode and an operating parameter for the power unit based on the DC
bus voltage.
FIG. 3 is a schematic functional diagram of a power controller
according to an exemplary embodiment of the present disclosure.
Referring to FIG. 3, the power controller 221 may include:
a detector 2211, configured to detect the DC bus voltage;
a calculator 2212, configured to determine a power reference value
based on the DC bus voltage; and
a controller 2213, configured to control an operating state of the
power unit 222 to adjust, in real time, the output power or the
absorbing power of the corresponding power unit based on the power
reference value.
FIG. 4 is a schematic diagram of determining a power reference
value by a calculator according to an exemplary embodiment of the
present disclosure. Referring to FIG. 4, when the energy storage
unit is configured to output power to the DC bus (i.e., the output
power is a positive value), the calculator may determine the output
power reference value based on the DC bus voltage in response to
real-time variation of the DC bus voltage.
For example, the power reference value is determined based on a
first preset operating mode when the DC bus voltage is greater than
a first preset value Vref1 and less than or equal to a second
preset value Vref2.
The power reference value is determined based on a first function
P=F.sub.1(V) when the DC bus voltage is greater than the second
preset value Vref2 and less than or equal to a third preset value
Vref3.
The power reference value is determined based on a third function
P=F.sub.3(V) when the DC bus voltage is greater than a fourth
preset value Vref4 and less than or equal to the first preset value
Vref1:
wherein V represents the DC bus voltage, and P represents the
output power reference value of the energy storage unit.
The first preset operating mode may refer to a mode where the
output power is in a common interval. That is, each power converter
determines a power output value based on its own energy management
demand. The first preset operating mode may be an equipower output
operating mode, or may be an operating mode of an output power
determined based on a function, which is not limited in the present
disclosure. It may be understood that when the DC bus voltage is
greater than the first preset value Vref1 and less than or equal to
the second preset value Vref2, excluding the voltage master
controller 21, each energy storage unit does not respond to or
control variation of the DC bus voltage.
A voltage range between the first preset value and the second
preset value is nearby the rated operational voltage of the DC bus,
and may be referred to as a "central section". In an exemplary
embodiment of the present disclosure, a relationship between the
first preset value Vref1 and the second preset value Vref2 may, for
example, satisfy the following formulas: Vref1=(1+K1)*Vp (1)
Vref2=(1+K2)*Vp (2)
wherein Vp represents a rated operational voltage of the DC bus, K1
and K2 represent any one value ranging from -30% to 30%, and K2 is
greater than K1. In general, the Vref2 is greater than the Vref1.
In some other embodiments, K1 and K2 also may be other values,
which may be set by those skilled in the art according to actual
situation.
A voltage range between the second preset value and the third
preset value and a voltage range between the fourth preset value
and the first preset value may be referred to as "operating
regulation section". In an exemplary embodiment of the present
disclosure, a relationship between the third preset value Vref3 and
the fourth preset value Vref4 may satisfy, for example, following
formulas: Vref3=(1+K3)*Vref2 (3) Vref4=(1-K4)*Vref1 (4)
wherein K3 and K4 represent any value ranging from 0 to 30%.
That is, in the embodiments of the present disclosure, Vref3 is
greater than Vref2, and Vref4 is smaller than Vref1. Similar to K1
and K2, values of K3 and K4 also may be set by those skilled in the
art according to the actual situation.
In the voltage range between Vref3 and Vref2, the DC bus voltage
exceeds the rated operational voltage, and its excess magnitude is
determined by the values of Vref3 and Vref2. In this interval, the
power reference value may be determined based on the first function
P=F1(V), wherein the first function may be a monotone decreasing
function, such that the controller 2213 may control the output
power of the power unit 222 to be inversely proportional to the
power of the DC bus voltage when the DC bus voltage is higher. That
is, the output power of the power unit 222 is reduced when the DC
bus voltage rises. In one embodiment, when the DC bus voltage is
greater than the third preset value Vref3, the power reference
value may be determined as a minimum preset output power, such that
the power unit 222 operates at the minimum preset output power,
thereby ensuring stable power supply of a DC grid as much as
possible when the DC bus voltage exceeds the third preset
value.
In the voltage range between Vref1 and Vref4, the DC bus voltage is
lower than the rated operational voltage, and its shortage
magnitude is determined by the values of Vref1 and Vref4. In this
interval, the power reference value may be determined based on the
third function P=F.sub.3(V), wherein the third function may be a
monotone decreasing function, such that the controller 2213 may
control the output power of the power unit 222 to be inversely
proportional to the voltage of the DC bus voltage when the DC bus
voltage is lower. That is, the output power of the power unit 222
is increased when the DC bus voltage drops. In one embodiment, when
the DC bus voltage is smaller than or equal to the fourth preset
value Vref4, the power reference value may be determined as a rated
output power, thereby ensuring stable power supply of a DC grid
when the DC bus voltage is lower than the fourth preset value.
In an exemplary embodiment of the present disclosure, the DC bus
voltage control system further includes a new energy generating
apparatus coupled to the DC bus, and in this case, the calculator
is further configured to:
limit the power reference value of the new energy generating
apparatus to an output power limit when the DC bus voltage is
greater than the third preset value, wherein the output power limit
is preset based on the fifth function corresponding to the DC bus
voltage. That is, when the DC bus voltage is less than or equal to
the third preset value, the new energy generating apparatus may be
controlled to operate at a maximum tracking power to supply power
to the DC grid. However, when the DC bus voltage is greater than
the third preset value, the DC grid is prone to excessive power.
Therefore, a power limit may be set for the new energy generating
apparatus based on the fifth function and the DC bus voltage, such
that its output power is not greater than the power limit. In this
way, the risk of excessive power of the DC grid is reduced, thereby
ensuring stable power supply of the DC grid.
Still referring to FIG. 4, in an exemplary embodiment of the
present disclosure, when the energy storage unit 22 is configured
to absorb power from the DC bus (i.e., the output power is a
negative value), the calculator 222 may determine the absorbing
power reference value based on the DC bus voltage in response to
real-time variation of the DC bus voltage.
For example, the power reference value is determined based on a
second preset operating mode when the DC bus voltage is greater
than the first preset value Vref1 and less than or equal to the
second preset value Vref2.
The power reference value is determined based on the second
function P=F.sub.2(V) when the DC bus voltage is greater than the
second preset value Vref2 and less than or equal to the third
preset value Vref3.
The power reference value is determined based on a fourth function
P=F.sub.4(V) when the DC bus voltage is greater than the fourth
preset value Vref4 and less than or equal to the first preset value
Vref1;
wherein V represents the DC bus voltage, and P represents the
absorbing power reference value of the energy storage unit.
Similar to the output power, when the DC bus voltage is near the
rated operational voltage, each power unit may be controlled to
determine a power absorption value based on their energy management
demands. When the DC bus voltage is greater than, within a certain
range, the rated operational voltage, and exceeds the second preset
value but does not exceed the third preset value, the absorbing
power reference value may be determined based on the second
function. In this embodiment, a negative number is employed to
represent the power absorbed from the DC bus, and a positive number
is employed to represent the power outputted to the DC bus. In an
exemplary embodiment of the present disclosure, the second function
is a monotone decreasing function. That is, when the DC bus voltage
is greater than, within a certain range, the rated operational
voltage, but does not exceed the third preset value Vref3, the
power value (its absolute value is taken if the power value is
negative) absorbed by the energy storage unit 22 is controlled to
be associated with the DC bus voltage, such that the power absorbed
by the power unit 222 is increased when the DC bus voltage rises.
In an exemplary embodiment of the present disclosure, when the DC
bus voltage is greater than the third preset value Vref3, the power
reference value also may be determined as the maximum preset
absorbing power, such that the power unit 222 absorbs power from
the DC bus at the maximum preset absorbing power. In this way,
power supply of a DC grid is ensured to be stable when the DC bus
voltage exceeds the third preset value.
When the DC bus voltage is less than, within a certain range, the
rated operational voltage, and is lower than the first preset value
but is not less than the fourth preset value, the absorbing power
reference value may be determined based on the fourth function. In
an exemplary embodiment of the present disclosure, a negative
number is employed to represent the power absorbed from the DC bus,
and thus the fourth function is a monotone decreasing function.
That is, when the DC bus voltage is smaller than, within a certain
range, the rated operational voltage, but is smaller than the
fourth preset value Vref4, the power value absorbed by the energy
storage unit 22 is controlled to be associated with the DC bus
voltage, such that the power absorbed by the power unit 222 is
decreased when the DC bus voltage drops. In an exemplary embodiment
of the present disclosure, when the DC bus voltage is smaller than
the fourth preset value Vref4, the power reference value also may
be determined as zero, such that the power unit 222 stops absorbing
power from the DC bus, thereby ensuring stable power supply of a DC
grid when the DC bus voltage is lower than the fourth preset
value.
In an exemplary embodiment of the present disclosure, the DC bus
voltage control system further includes DC loads and/or AC loads, a
part of which may be controlled to be partially unloaded based on
the bus voltage when the DC bus voltage is less than or equal to
the fourth preset value, such that power balance is achieved, or
all the DC loads and/or AC loads are unloaded to go into a
protection mode. That is, when the DC bus voltage is less than or
equal to the fourth preset value, the DC loads and/or the AC loads
may be controlled to be unloaded by themselves.
In this embodiment, the first preset value, the second preset
value, the third preset value, and the fourth preset value are
described in one figure. However, in actual operation, when the
energy storage unit is configured to absorb power from the DC bus
or output power to the DC bus, the reference preset values may be
different. The reference preset values of the new energy generating
apparatus, the DC loads and the AC loads may be different from
those of the energy storage units. In some embodiments, because
each energy storage unit has its own controller and calculator, the
first preset value, the second preset value, the third preset value
and the fourth preset value different from those of other energy
storage units may be set for each energy storage unit to flexibly
adjust the working power based on its own working conditions and
energy management demands so as to maintain stable power supply of
a DC grid.
In addition, the first preset value, the second preset value, the
third preset value and the fourth preset value above are merely
exemplary descriptions. In practical applications, a tolerance may
be set for each preset value, and when the DC bus voltage is within
the tolerance range of an xth preset value, a power determination
rule that the calculator should refer to is determined according to
the actual situation.
By enabling each energy storage unit, the new energy generating
apparatus and the loads adjust operating power thereof based on the
DC bus voltage, the power of the DC grid may be flexibly adjusted.
In this way, adverse effects of communication conditions on
adjustment of the DC bus voltage can be reduced, thereby ensuring
the reliability of power supply for the DC gird.
It is to be noticed that although a plurality of modules or units
of the device for action execution have been mentioned in the above
detailed description, this partition is not compulsory. Actually,
according to the embodiment of the present disclosure, features and
functions of two or more modules or units as described above may be
embodied in one module or unit. Reversely, features and functions
of one module or unit as described above may be further embodied in
more modules or units.
FIG. 5A is a flowchart of a method for controlling a DC bus voltage
according to an embodiment of the present disclosure. The method
for controlling a DC bus voltage may be applied to the system for
controlling a DC bus voltage as shown in FIG. 2.
Referring to FIG. 5A, the method 500A for controlling a DC bus
voltage may include:
Step S1A: detecting a DC bus voltage;
Step S2A: determining, by the energy storage unit, a power
reference value based on the DC bus voltage;
Step S3A adjusting, by the energy storage unit, one of output power
and absorbing power based on the power reference value.
FIG. 5B is a flowchart of a method for controlling a DC bus voltage
according to another embodiment of the present disclosure. The
method for controlling a DC bus voltage may be applied to the
system for controlling a DC bus voltage as shown in FIG. 2.
Referring to FIG. 5B, the method 500 for controlling a DC bus
voltage may include:
Step S1: detecting a DC bus voltage;
Step S2: outputting or absorbing power by the energy storage unit
based on a preset mode when the DC bus voltage is greater than a
first preset value and less than or equal to a second preset
value;
Step S3: outputting power based on a power value corresponding to a
first function and the DC bus voltage or absorbing power based on a
power value corresponding to a second function and the DC bus
voltage by the energy storage unit when the DC bus voltage is
greater than the second preset value and less than or equal to a
third preset value; and
Step S4: outputting power based on a power value corresponding to a
third function and the DC bus voltage or absorbing power based on a
power value corresponding to a fourth function and the DC bus
voltage by the energy storage unit when the DC bus voltage is
greater than a fourth preset value and less than or equal to the
first preset value.
In an exemplary embodiment of the present disclosure, the method
500 may further include:
Step S5: the energy storage unit running at a minimum preset output
power or a maximum preset absorbing power when the DC bus voltage
is greater than the third preset value, and
Step S6: the energy storage unit outputting power at rated power or
stopping absorbing power when the DC bus voltage is less than or
equal to the fourth preset value.
In an exemplary embodiment of the present disclosure, the DC bus
system further includes a new energy generating unit coupled to the
DC bus.
An output power limit of the new energy generating unit is set
based on a power value in a fifth function corresponding to the DC
bus voltage when the DC bus voltage is greater than the third
preset value to control an output power of the new energy
generating apparatus not more than the output power limit.
In an exemplary embodiment of the present disclosure, the DC bus
system further includes a DC load or an alternating current (AC)
load coupled to the DC bus.
The DC load or the AC load is unloaded when the DC bus voltage is
less than or equal to the fourth preset value.
In an exemplary embodiment of the present disclosure, independent
variables of the first function and the third function are the DC
bus voltage, dependent variables of the first function and the
third function are an output power reference value of the energy
storage unit, and the energy storage unit controls, based on the
output power reference value, power outputted from the energy
storage unit to the DC bus.
In an exemplary embodiment of the present disclosure, independent
variables of the second function and the fourth function are the DC
bus voltage, dependent variables of the second function and the
fourth function are an absorbing power reference value of the
energy storage unit, and the energy storage unit controls, based on
the absorbing power reference value, power absorbed from the DC bus
by the energy storage unit.
In an exemplary embodiment of the present disclosure, an
independent variable of the fifth function is the DC bus voltage,
and a dependent variable of the fifth function is the output power
limit of the new energy generating unit. The new energy generating
unit controller controls that, based on the output power limit, the
power outputted to the DC bus by the new energy generating
apparatus is below the output power limit.
In an exemplary embodiment of the present disclosure, a positive
number is adopted to represent a power outputted to the DC bus, and
a negative number is adopted to represent a power absorbed from the
DC bus. The first function, the second function, the third
function, the fourth function and the fifth function are monotone
decreasing functions.
In the embodiment as shown in FIG. 5B, functions represented by the
second function and the third function are different from the
embodiment as shown in FIG. 4, which are set for ease of
description but have no tangible impact on implementation manners
of the embodiments of the present disclosure.
The embodiments of the present disclosure are described below with
reference to application scenes.
FIG. 6 is a schematic operating diagram when the embodiments of the
present disclosure are applied to a braking energy absorption
system of a subway substation. Referring to FIG. 6, supposing that
a subway traction power supply system having a rated voltage of
750V is equipped with a 1 MW feed inverter (ERS) and a 500 kW
energy storage system (ESS), the subway traction power supply
system continuously run without communication and system control
commands.
The first preset value is set to 800V, the second preset value is
set to 850V, the third preset value is set to 900V. and the fourth
preset value is set to 750V. The range between the first preset
value and the second preset value is defined as a first voltage
range, the range between the second preset value and the third
preset value is defined as a second voltage range, and a range
between the first preset value and the fourth preset value is
defined as a third voltage range.
It is assumed that the operating powers of the energy storage
system (ESS) in the second voltage range and the third voltage
range are determined by functions represented by line segments (1)
and (2) in FIG. 6 respectively:
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times. ##EQU00001##
wherein, Vds represents the first preset value, Vss represents the
second preset value, Vsm represents the third preset value, Vdm
represents the fourth preset value, P0 represents a corresponding
power value when the bus voltage is the second preset value, Psm
represents a corresponding power value when the bus voltage is the
third preset value, and Pdm represents a corresponding power value
when the bus voltage is the fourth preset value.
It is assumed that the operating powers of the feed inverter
(ERS)/rectifying unit in the second voltage range and the third
voltage range are determined by functions represented by line
segments (3) and (4) in FIG. 6 respectively:
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times.
##EQU00002##
wherein, Psm2 represents a corresponding power value when the bus
voltage of the feed inverter is the third preset value, and Pdm2
represents a corresponding power value when the bus voltage of the
rectifying unit is the fourth preset value.
The functional relationship of P1-P4 may be preset in the
calculator of each energy storage unit (ESS. ERS, and the
rectifying unit), such that its own control power of the controller
of each energy storage unit may be independently determined by the
DC bus voltage, and the operating power of the corresponding energy
storage unit is controlled based on the control power.
Specific implementation methods for power control may have various
forms. For example, in this embodiment, this may be implemented by
current control on the side of the energy storage unit.
As shown in FIG. 7, a detector of each energy storage unit detects
the DC bus voltage. The calculator obtains the control power P*
based on the DC bus voltage Vbus and the above preset functional
relationship P(V), and then the operating current I* of the energy
storage unit is determined based on the control power P* and the
operating voltage VB. The controller controls the current of the
energy storage unit based on a current controller and the operating
current I*, such that the power outputted or absorbed by the energy
storage unit is consistent with the control power P*. In other
applications, the power control of the energy storage unit also may
be implemented by means of current control on the DC bus side.
Referring to FIG. 8A-FIG. 8C, the exemplary running process of the
system as shown in FIG. 6 is as below.
As shown in FIG. 8A, when it is detected that the DC bus voltage is
within the first voltage range (800V to 850V), the controller of
the energy storage converter controls the power unit to stop
outputting power, i.e., not responding to voltage fluctuation.
When it is detected that the DC bus voltage is within the second
voltage range (850V to 900V), it is assumed that the initial system
does not have the energy storage converter (ESS), only the inverter
and a resistive load absorb the braking energy, and the power
control of the inverter allowed for control operation based on a
preset functional relationship. When 1000 KW net braking energy is
injected into the DC grid, the DC bus voltage rises to 900V until
the feed inverter operates at -1000 KW under full load to cancel
the net injection of the braking energy. When 1500 KW braking
energy is injected into the grid, the DC bus voltage is as high as
900V. At this moment, a load resistor must be started to absorb
excessive braking energy.
As shown in FIG. 8B, if a set of 500 kW ESS is equipped in the
system, the operating power of the ESS is controlled based on the
preset functional relationship. In this way, plug and play may be
implemented without changing other settings of the system. When
1000 KW net braking energy is injected into the DC grid, the ESS
controller and the inverter determine the absorbing power based on
the DC bus voltage, such that the DC bus voltage stabilizes at
V_IM, lower than 900V. When 1500 KW braking energy is injected into
the grid, the DC bus voltage reaches 900V, but it is still not
required to start the load resistor to absorb the braking energy.
In this way, by performing power control on the ESS based on the
method according to the present disclosure, rise of the DC bus
voltage caused by subway braking is effectively restrained.
As shown in FIG. 8C, when it is detected that the DC bus voltage is
within the third voltage range (800V to 750V), the calculator of
the ESS may determine, based on the DC bus voltage, that the output
power is from 0 to 500 kW. The controller controls the power unit
of the ESS to compensate the power for the rectifier. In this way,
drop of the DC bus voltage caused by acceleration of a train may be
reduced. When the traction load is 1000 kW, the ESS discharge power
adopting the method according to this embodiment is 350 kW, the
rectification output power is 650 kW, and the bus voltage is close
to 770V. In this way, by independently controlling the power
operation based on the DC bus voltage, the energy storage unit
reduces drop of the voltage of the DC grid in the starting process
of a train.
FIG. 9 is a schematic diagram according to still another embodiment
of the present disclosure, i.e., a block diagram of a DC grid
having a DC bus new energy generating apparatus and an energy
storage unit.
Referring to FIG. 9, it is assumed that the new energy generating
apparatus coupled to the DC bus and a plurality of energy storage
units are connected to run. It is assumed that a DC bus
photovoltaic power generation battery energy storage AC grid-tied
power generation system has a rated DC bus voltage of 800V, and the
system is equipped with a 100 kW grid-tied inverter, a 100 kW
energy storage system (Battery1/ESS1), and a 50 kW energy storage
system (Battery2/ESS2), a 100 kW new energy generating apparatus
and a DC load, wherein the new energy generating apparatus includes
a photovoltaic panel PV and a DC/DC converter thereof.
In a grid-tied mode, an operation mode of a preset grid-tied
inverter is to control the DC bus voltage within the first voltage
range and a tolerance thereof. At this moment, a preset mode of a
DC/DC converter of the new energy generating apparatus is a maximum
power point tracking mode (MPPT mode). Power is injected into the
DC bus according to illumination conditions, and each energy
storage system performs charging and discharging operation or
standby according to an energy management strategy of the preset
mode. When the grid is abnormal, the grid-tied inverter goes into a
stand alone mode according to the preset mode. Under any one of the
above operating conditions, the system bus voltage may be
automatically controlled based on the method of the present
disclosure.
When the grid is normal, the grid-tied inverter controls the bus
voltage within the first voltage range, and injection power of the
inverter (a direction of injection to the DC bus is positive, and a
reverse direction is negative) is between 100 kW and -100 kW. If
the DC power other than that of the inverter (for example, the sum
of the DC power of the new energy generating apparatus, the battery
and the DC load) does not exceed the inverter power range limit,
the DC bus voltage can be maintained within the first voltage
range. If the DC power other than that of the inverter exceeds the
inverter power limit, the DC bus voltage cannot be maintained
within the first voltage range, and may likely go into the second
voltage range or the third voltage range.
At this moment, if the DC bus voltage goes into the second voltage
range, this indicates that the power (negative power) absorbed from
the DC bus by the inverter is limited, which is insufficient to
balance and counteract the injection power of the new energy
generating apparatus and the energy storage unit, thus resulting in
rise of the bus voltage. In this embodiment, each energy storage
unit may automatically reduce the output power or increase the
absorbing power as the DC bus voltage increases based on the
control method of the present disclosure, such that the DC bus
voltage reaches a new balance. If the absorbing power of the energy
storage unit reaches the maximum value, but the power of the DC bus
still exceed the absorbing capacity of the grid-tied inverter after
the power of the new energy generating apparatus is injected into
the DC bus, the DC bus voltage may rise to go into the fourth
voltage range (more than the third preset value). At this moment,
the new energy generating apparatus may automatically reduce the
output power, such that the output power does not exceed the
maximum preset value, thereby ensuring that the DC bus voltage
reaches a new balance.
If the power inputted from the inverter to the DC bus is still
insufficient to compensate the power loss of the DC bus, the DC bus
voltage goes into the third voltage range, and each energy storage
unit may automatically increase the output power to maintain the
balance of the DC bus voltage.
Referring to FIG. 10, it is assumed that the operating power of
energy storage units ESS1 and ESS2 may be determined by the
function curves as shown in (1), (2) (3) and (4). Within the second
voltage range, formulas of the Functions (1) and (3) may be as
below:
.times..times..times..times..times..times..times..times..times..times.
##EQU00003##
The power control of the new energy generating apparatus is
determined by the function curve as shown in (5). Within the fourth
voltage range, the formula of the Function (5) may be as below:
.times..times..times..times..times..times..times..times.
##EQU00004##
As can be seen from FIG. 10, when the DC bus voltage rises between
Vss (840V) and Vsm (870V), the charge power of the ESS1 (a 100 kW
energy storage unit) correspondingly increases from 0 W to 100 kW.
When the DC bus voltage drops between 750V and 800V, the discharge
power of the ESS1 correspondingly increases from 0 W to 100 kW. The
operation rule of the ESS2 (a 50 kW energy storage unit) is similar
to that of the ESS1, and thus its detailed description is omitted
here. To suppress excessive bus voltage, within the fourth voltage
range, the controller of the new energy generating apparatus limits
the power output based on a V-P function corresponding to the fifth
function, which may be set by the those skilled in the art
according to the actual situation.
In addition, if the grid changes from a normal condition to an
abnormal condition, the inverter may run independently from the
grid and absorb power from the DC bus to supply power to the AC
load. If the DC bus power is out of balance, the DC bus voltage may
likely go into the second voltage range or the third voltage
range.
In the stand alone mode, the DC bus voltage goes into the second
voltage range, which indicates that power consumed by the AC load
and the DC load is insufficient to counteract the injection power
of the new energy generating apparatus and each energy storage
unit, thereby causing rise of the bus voltage. In the embodiments
of the present disclosure, each energy storage unit may
automatically reduce the output power or increase the absorbing
power as the voltage increases, such that the DC bus power reaches
a new balance. If there is still excessive PV power exceeding the
absorbing capacity of the grid-tied inverter when the absorbing
power of the energy storage unit reaches the maximum preset value,
the bus voltage may rise to go into the fourth voltage range, and
the converter of the new energy generating apparatus may
automatically reduce the output power, such that the DC bus voltage
reaches a new balance.
Conversely, in the stand alone mode, the DC bus voltage goes into
the third voltage range, which indicates that the load power of the
DC bus exceeds the injection power of the new energy generating
apparatus or the energy storage unit, thereby causing drop of the
bus voltage. In the embodiments of the present disclosure, at this
moment, the new energy generating apparatus may track the output at
the maximum power, and each energy storage unit may automatically
increase the output power or reduce the absorbing power as the
voltage decreases, such that the DC bus power reaches a new
balance. If the output power of the energy storage unit reaches its
maximum allowable value but is still insufficient to compensate the
power absorbed by the load, the DC bus voltage may drop and go into
the fifth voltage range. At this moment, each AC load or DC load
may be partially unloaded by themselves based on the bus voltage to
reach a power balance, or may be completely unloaded by themselves
to go into a protection mode.
As shown in the ESS1 and ESS2 power control functions as shown in
FIG. 10, if there exist a plurality of energy storage units having
different maximum powers in the system, a power control function
may be separately set for each energy storage unit or even
different voltage ranges may be defined to achieve parallel
operation. In addition, strategies in response to DC bus voltage
deviation also may be independently set for each energy storage
unit, for example, in response to a starting point, a terminal
point, a function curve or a look-up table and so on, such that
priority and charge and discharge energy management may be
respectively set, thereby avoiding complex parameter debugging and
procedure processing in centralized control of the system,
communication link establishment or multi-loop feedback control and
so on, making the energy storage unit easy to implement modular
expansion and plug and play.
In the embodiments of the present disclosure, each energy storage
unit coupled to the DC bus, the new energy generating apparatus and
the load arranged in the system respectively adjust their own
operating powers based on the DC bus voltage, such that the DC bus
voltage may reach a new balance. In this way, the system voltage
remains stable without communication connection or centralized
control instructions, thereby avoiding frequently switching system
states. Each energy storage unit determines its own operating power
by means of calculation or look-up table merely based on the DC bus
voltage, which avoids complicated control modes such as double-loop
or multi-loop feedback control, simplifies the control process and
improves reliability.
As will be appreciated by one skilled in the art, aspects of the
present disclosure may be embodied as a system, method or program
product. Accordingly, aspects of the present disclosure may take
the form of an entirely hardware embodiment, an entirely software
embodiment (including firmware, micro-code, etc.) or an embodiment
combining software and hardware aspects that may all generally be
referred to herein as a "circuit," "module" or "system."
Moreover, the above accompanying drawings are merely illustrative
description of processes included in the method according to the
exemplary embodiments of the present disclosure and are not
intended to limit the present disclosure. It is easy to understand
that the processes shown in the above accompanying drawings do not
indicate or limit time sequences of these processes. Furthermore,
it is also easy to understand that these processes may be executed,
for example, synchronously or asynchronously in a plurality of
modules.
Other embodiments of the present disclosure will be apparent to
those skilled in the art from consideration of the specification
and practice of the invention disclosed here. This application is
intended to cover any variations, uses, or adaptations of the
present disclosure following the general principles thereof and
including such departures from the present disclosure as come
within known or customary practice in the art. It is intended that
the specification and embodiments be considered as exemplary only,
with a true scope and spirit of the present disclosure being
indicated by the claims.
* * * * *